Detectable warning surfaces, such as present on walkways are known.
Federal and state regulations have required such for ten or more years.
Examples include my earlier U.S. Pat. No. 5,271,690 to Fennessy, Sr., U.S. Pat. No. 5,061,172 to Fennessy, Sr., and U.S. Pat. No. Des. 338,817 to Fennessy, Sr.
Each of those earlier patents of mine is incorporated herein by reference.
Additional patents of interest include U.S. Pat. No. 5,219,511 to Fennessy, Sr. and U.S. Pat. No. 6,444,077 to Fennessy.